To initiate the establishment of a communication link with an access node, a wireless device may perform an initial synchronization process and a random access procedure. To perform the random access procedure, the wireless device typically selects a random access (RACH) preamble, and the wireless device can use the selected RACH preamble to transmit an initial access request to the access node. The randomly selected preamble can distinguish the wireless device's initial access request from other similar requests from other wireless devices. Access node preambles can be determined based on a unique sequence such as a root sequence. The same root sequence is typically not assigned to more than one access node in proximity to one other access nodes, to mitigate confusion among initial access requests received by the access nodes.
Preambles for each access node can then be obtained based on, for example, a cyclic shift of the assigned root sequence. The cyclic shift value can depend on the size of the coverage area of each access node, as well as other factors such as delay spread factors. In many cases, the larger the coverage area of the access node, the larger the cyclic shift which is required. Because root sequences cannot be assigned to more than one access node in relative proximity to other access nodes, root sequence assignment is an important consideration in network configuration.
Overview
In operation, a first coverage radius of a first access node is determined, as well as a first neighbor list of the first access node. The first neighbor list comprises second access nodes which are each a neighbor access node of the first access node. A second coverage radius and a second neighbor list of each of the second access nodes is determined. An access node comprising a largest coverage radius from among the first access node and the second access nodes is selected. A number of root sequences required for the selected access node is calculated based on the coverage radius of the selected access node, and root sequences are assigned to the selected access node according to the number of root sequences required.